19.11
Somatosensation
The somatosensory system relays sensory information from the skin, mucous membranes, limbs, and joints. Somatosensation is more familiarly known as th…
During somatosensation, the sense of touch, sensory neurons within the skin first detect mechanical disturbance via mechanoreceptors.
If the stimuli are light vibrations, Meissner's corpuscles respond. With an added constant presence, Merkel discs react, since they adapt slowly. Ruffini endings, also slow to adapt, respond to deeper, sustained stimuli. If this becomes repetitive, it is detected by Pacinian corpuscles.
Following detection, sensations are transduced into electric signals, and travel towards the spinal cord, specifically the dorsal root ganglion, where processes from the same area of the body, called a dermatome, make up a single dorsal root. Continuing on to the brain stem, the medulla in particular, the sensory information decussates, meaning it crosses over to the other side before being sent to the thalamus. From here, the message is finally sent to the somatosensory cortex.
This cortical region contains a map of the body called a homunculus. Because of decussation, the left side of the body is represented in the right cortex and vice versa.
Fingers occupy a large cortical area because they have a high density of mechanoreceptors with small receptive fields, which gives them better stimulus discrimination. In contrast, the back has fewer receptors and larger receptive fields, and thus a smaller cortical representation.
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Q1: What are the four main types of mechanoreceptors in hairless skin?
The four main mechanoreceptors are Meissner corpuscles, Merkel disks, Ruffini endings, and Pacinian corpuscles. Meissner corpuscles and Pacinian corpuscles are rapidly adapting and detect dynamic stimuli like vibrations. Merkel disks and Ruffini endings are slow adapting and respond to sustained touch and pressure.
Q2: How does the somatosensory pathway transmit information from skin to brain?
Mechanoreceptors in skin detect stimuli and send signals through primary neurons to the dorsal root ganglion. Secondary neurons in the medulla decussate, crossing to the opposite side before reaching the thalamus. Tertiary neurons then carry the signal to the somatosensory cortex, where the body is mapped as a homunculus.
Q3: Why do fingers have larger cortical representation than the back?
Fingers have a high density of mechanoreceptors with small receptive fields, enabling better stimulus discrimination. This dense receptor distribution requires more cortical area for processing fine sensory details. In contrast, the back has fewer receptors and larger receptive fields, resulting in smaller cortical representation.
Q4: What is decussation and why does it occur in the somatosensory pathway?
Decussation is the crossing of sensory information from one side of the body to the opposite side of the brain. This occurs in the medulla when secondary neurons cross over before sending signals to the thalamus. As a result, the left side of the body is processed in the right cortex and vice versa.
Q5: What is a dermatome and how does it relate to the dorsal root ganglion?
A dermatome is a designated area of skin innervated by sensory neurons from a single dorsal root ganglion. Primary neurons with cell bodies in the dorsal root ganglia extend axons to specific dermatomes. Each dermatome represents a distinct region where sensory information is initially detected before traveling to the spinal cord.
Q6: How can the somatosensory cortex reorganize after injury or specialized use?
The somatosensory cortex undergoes cortical reorganization when body regions do not function as intended. Braille readers develop larger finger representations in the cortex due to intensive tactile training. In forearm amputees, the cortical region previously connected to the amputated arm can be remapped to adjacent regions, sometimes causing phantom limb sensations.
Q7: How do slow-adapting and rapidly-adapting mechanoreceptors differ in function?
Slow-adapting mechanoreceptors like Merkel disks and Ruffini endings continue responding to prolonged contact, detecting sustained touch and pressure. Rapidly-adapting mechanoreceptors like Meissner corpuscles and Pacinian corpuscles respond to dynamic stimuli such as vibrations and changing pressure. This functional difference allows the sensory system to detect both constant and changing tactile information.